Phase change memory that switches between crystalline phases

ABSTRACT

A phase change memory may transition between two crystalline states. In one embodiment, the phase change material is a chalcogenide which transitions between face centered cubic and hexagonal states. Because these states are more stable, they are less prone to drift than the amorphous state conventionally utilized in phase change memories.

BACKGROUND

This invention relates generally to phase change memories.

In conventional phase change memories, a memory material, such as achalcogenide, may switch between crystalline and non-crystalline orso-called amorphous phases. One problem with these transitions is thatparticularly the amorphous phase is not very stable. As a result, thecharacteristics of the device in the amorphous phase may drift.

Commonly, a phase change memory transitions between the amorphous phaseand a crystalline phase which may be a face centered cubic (FCC) phase.

Thus, there is a need for ways to transition phase change memoriesbetween states that are less subject to drift.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of one embodiment of the presentinvention;

FIG. 2 is an enlarged, partially cross-sectional and partially schematicview of one cell shown in FIG. 1 in accordance with one embodiment ofthe present invention;

FIG. 3 is a typical graph of conductivity versus annealing temperature;and

FIG. 4 is a system depiction of one embodiment of the present invention.

DETAILED DESCRIPTION

Turning to FIG. 1, an embodiment of a memory 100 is illustrated. Memory100 may include a 3×3 array of memory cells 111-119, wherein memorycells 111-119 each include a select device 120 and a memory element 130.Although a 3×3 array is illustrated in FIG. 1, the scope of the presentinvention is not limited in this respect. Memory 100 may have a largerarray of memory cells.

In one embodiment, memory elements 130 may comprise a phase changematerial. In this embodiment, memory 100 may be referred to as a phasechange memory. A phase change material may be a material havingelectrical properties (e.g. resistance, capacitance, etc.) that may bechanged through the application of energy such as, for example, heat,light, voltage potential, or electrical current. Examples of a phasechange material may include a chalcogenide material.

A chalcogenide alloy may be used in a memory element or in an electronicswitch. A chalcogenide material may be a material that includes at leastone element from column VI of the periodic table or may be a materialthat includes one or more of the chalcogen elements, e.g., any of theelements of tellurium, sulfur, oxygen, or selenium.

Memory 100 may include column lines 141-143 and row lines 151-153 toselect a particular memory cell of the array during a write or readoperation. Column lines 141-143 and row lines 151-153 may also bereferred to as address lines since these lines may be used to addressmemory cells 111-119 during programming or reading. Column lines 141-143may also be referred to as bit lines and row lines 151-153 may also bereferred to as word lines.

Memory elements 130 may be connected to row lines 151-153 and may becoupled to column lines 141-143 via select device 120. While one selectdevice 120 is depicted, more select devices may also be used. Therefore,when a particular memory cell (e.g., memory cell 115) is selected,voltage potentials may be applied to the memory cell's associated columnline (e.g., 142) and row line (e.g., 152) to apply a voltage potentialacross the memory cell.

Series connected select device 120 may be used to access memory element130 during programming or reading of memory element 130. The selectdevice 120 can be a diode or a transistor, as two examples. Selectdevice 120 may also be referred to as an access device, an isolationdevice, or a switch.

Referring to FIGS. 1 and 2, a cell 115 may include a memory element 130and a select device 120. The select device 120 may, for example, be afield effect transistor or a diode. The select device 120 is coupled toa lower electrode 22 formed in an insulating layer 12. A memory material14 may be formed thereover. An upper electrode 20 may extendtransversely to the length of the lower electrode 22 in one embodiment.

The memory material may, for example, be a chalcogenide known as GST2,2,5, or Ge₂Sb₂Te₅, which is 20% germanium, 20% antimony, and 50%tellurium. In response to current-based Joule heating, the memorymaterial 14 transitions between two different crystalline states. In oneembodiment, the memory material 14 may transition between a moreresistive face centered cubic (FCC) crystalline state (indicated in FIG.3) and an even less resistive hexagonal state (also indicated in FIG.3).

In the hexagonal state, very low resistance and relatively highconductivity on the order of 500 to 2000 (ohm-cm)⁻¹ may be achieved asindicated in FIG. 3. Generally, the material 14 is heated to atemperature of above 350 degrees and held at that temperature forsufficient time to transition to the hexagonal state. Thus, in somecases, a relatively longer amount of time and more heat may be utilizedto transition to the hexagonal state. However, in the hexagonal state,the structure is extremely stable and drift is very low, or notmeasureable.

To transition back from the hexagonal state to the face centered cubicstate, a conventional reset operation may be utilized. In otherembodiments, it may be desirable to reset and then to transition to aface centered cubic state. Thus, a two-step reset may be requiredwherein the initial step is a reset as is conventionally done, followedby a set operation to transition the device to the face centered cubicstate.

In conventional phase change memories, the face centered cubic state mayalso be utilized, but it is the only crystalline state that is achieved.Generally, those devices transition between face centered cubic and anamorphous phase, neither of which is as stable and as drift resistant asthe hexagonal state.

Programming of phase change material 18 to alter the state or phase ofthe material may be accomplished by applying voltage potentials toconductive materials 22 and 20, thereby generating a voltage potentialacross the memory element 120 and select device 130. When the voltagepotential is greater than the threshold voltage of the memory element120, then an electrical current may flow through memory material 14 inresponse to the applied voltage potential, and may result in heating ofmemory material 14.

This heating may alter the memory state or phase of memory material 14.Altering the phase or state of memory material 14 may alter anelectrical characteristic of memory material 14, e.g., the resistance ofthe material may be altered by altering the phase of the memory material14.

In the “reset” state, memory material 14 may be in a face centered cubiccrystalline state and in the “set” state, memory material 14 may be inan a hexagonal crystalline state. The resistance of memory material 14in the face centered cubic state may be greater than the resistance ofmemory material 14 in the hexagonal crystalline state. It is to beappreciated that the association of reset and set with crystallinestates is a convention and that at least an opposite convention may beadopted.

Using electrical current, memory material 14 may be heated to arelatively higher temperature to “reset” memory material 14 (e.g.,program memory material 18 to a logic “0” value). Heating the volume ofmemory material 14 to a relatively higher crystallization temperaturemay crystallize memory material 18 and “set” memory material 14 (e.g.,program memory material 18 to a logic “1” value).

Conductive material 20, 22 may be a thin film material having athickness ranging from about 20 Å to about 2000 Å. In one embodiment,the thickness of the material 20, 22 may range from about 100 Å to about1000 Å. In another embodiment, the thickness of the material 20, 22 maybe about 300 Å. Suitable materials may include a thin film of titanium(Ti), titanium nitride (TiN), titanium tungsten (TiW), carbon (C),silicon carbide (SiC), titanium aluminum nitride (TiAlN), titaniumsilicon nitride (TiSiN), polycrystalline silicon, tantalum nitride(TaN), some combination of these films, or other suitable conductors orresistive conductors compatible with material 14.

Turning to FIG. 3, a portion of a system 500 in accordance with anembodiment of the present invention is described. System 500 may be usedin wireless devices such as, for example, a personal digital assistant(PDA), a laptop or portable computer with wireless capability, a webtablet, a wireless telephone, a pager, an instant messaging device, adigital music player, a digital camera, or other devices that may beadapted to transmit and/or receive information wirelessly. System 500may be used in any of the following systems: a wireless local areanetwork (WLAN) system, a wireless personal area network (WPAN) system, acellular network, although the scope of the present invention is notlimited in this respect.

System 500 may include a controller 510, an input/output (I/O) device520 (e.g. a keypad, display), a memory 530, and a wireless interface 540coupled to each other via a bus 550. It should be noted that the scopeof the present invention is not limited to embodiments having any or allof these components.

Controller 510 may comprise, for example, one or more microprocessors,digital signal processors, microcontrollers, or the like. Memory 530 maybe used to store messages transmitted to or by system 500. Memory 530may also optionally be used to store instructions that are executed bycontroller 510 during the operation of system 860, and may be used tostore user data. Memory 875 may be provided by one or more differenttypes of memory. For example, memory 530 may comprise any type of randomaccess memory, a volatile memory, a non-volatile memory such as a flashmemory and/or a memory such as memory 100 discussed herein.

I/O device 520 may be used by a user to generate a message. System 500may use wireless interface 540 to transmit and receive messages to andfrom a wireless communication network with a radio frequency (RF)signal. Examples of wireless interface 540 may include an antenna or awireless transceiver, although the scope of the present invention is notlimited in this respect.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. a method comprising: transitioning a phase change memory between twocrystalline states.
 2. The method of claim 1 including heating saidphase change memory material to a temperature above 350° C.
 3. Themethod of claim 1 including transitioning a phase change memory to thehexagonal crystalline state.
 4. The method of claim 3 includingtransitioning a phase change memory between a face centered cubic andhexagonal crystalline states.
 5. The method of claim 4 includingtransitioning from the hexagonal crystalline state to the face centeredcubic state by resetting said memory and then programming said memory tothe face centered cubic state by applying heat.
 6. A phase change memorycomprising: a pair of conductors; and a phase change memory material totransition between crystalline states, said material located betweensaid electrodes.
 7. The memory of claim 6 wherein said material is achalcogenide.
 8. The memory of claim 7 wherein said material includesGe₂Sb₂Te₅.
 9. The memory of claim 6 wherein said device to transition tothe hexagonal crystalline state.
 10. The memory of claim 8 wherein saiddevice to transition between hexagonal and face centered cubiccrystalline states.
 11. The memory of claim 6 including a select device.12. A system comprising: a phase change memory to transition between twocrystalline states; a controller coupled to said phase change memory;and a wireless interface coupled to said controller.
 13. The system ofclaim 12 wherein said memory includes a chalcogenide material.
 14. Thesystem of claim 13 wherein said material is Ge₂Sb₂Te₅.
 15. The system ofclaim 12 wherein said memory to transition to the hexagonal crystallinestate.
 16. The system of claim 14 wherein said memory to transitionbetween hexagonal and face centered cubic crystalline states.
 17. Thesystem of claim 12 wherein said memory includes a select device coupledto a phase change memory element.
 18. The system of claim 17 whereinsaid memory includes a pair of conductors sandwiching a chalcogenidematerial.